FIG. 1 illustrates a conventional power transfer system 100 that selectively transfers power from an adapter connector 102A or a universal serial bus (USB) connector 102U to a load 108. More specifically, the system 100 includes a switch 106A coupled between the adapter connector 102A and the load 108, and a switch 106U coupled between the USB connector 102U and the load 108. If a USB power source 134U is plugged into the USB connector 102U and there is no adapter plugged into the adapter connector 102A, then the system 100 transfers power from the USB connector 102U to the load 108 by turning on the switch 106U and turning off the switch 106A. If an adapter 134A is plugged into the adapter connector 102A when the USB power source is powering the load 108, then the system 100 selects the adapter 134A to power the load 108 by turning on the switch 106A and turning off the switch 106U. The conventional power transfer system 100 has some shortcomings.
As shown in FIG. 1, each of the switches 106A and 106U includes a body diode having an anode coupled to the corresponding connector 102A or 102U and having a cathode coupled to a connection node 110 of a filter capacitor 104 and the load 108. When no power source is powering the load 108, the connection node 110 has a low voltage, e.g., zero volts. Thus, plugging a power source into the connector 102A or 102U may cause the body diode of the corresponding switch 106A or 106U to be turned on automatically, and there may be a voltage jump at the connection node 110 and a large transient current flowing through the corresponding switch 106A or 106U to charge the filter capacitor 104. This may cause damage to the corresponding switch 106A or 106U and the corresponding connector 102A or 102U.
Additionally, if a power source is plugged into the connector 102A or 102U, and the power source has a poor contact with the connector, then voltage spikes and/or current spikes may appear at the connection node 110, which may also cause damage to the system 100.
Furthermore, a voltage output from an adapter may be greater than a voltage output from a USB power source. If an adapter 134A having a higher output voltage is plugged into the adapter connector 102A when a USB power source 134U having a lower output voltage is powering the load 108 through the USB connector 102U, then it may cause a voltage jump at the connection node 110. The voltage jump can be relatively large if a difference between the output voltages of the adapter 134A and the USB power source 134U is relatively large, which may also cause damage to the system 100.